1. Field of the Invention
The present invention relates to a coordinate input device that detects an indication point on a screen, and a display apparatus having the same. More particularly, it is relates to a technique which is effective in increasing the coordinate detection precision of a display apparatus having a capacitive coupling type coordinate input device.
2. Description of the Related Art
A display apparatus having a detection device (hereinafter, also called a touch sensor or a touch panel) having a screen input function for inputting information by a touch operation (contact pressing operation, hereinafter, simply called a touch) using a finger or the like of a user on the display screen is used for mobile electronic devices such as PDAs and portable terminals, various electronic appliances, and stationary customer guide terminals such as unattended reception machines. As such a touch input device, a resistive type that detects a change in resistance of a touched portion, a capacitive coupling type that detects a change in capacitance, an optical sensor type that detects a change in light intensity of a portion blocked by a touch, and the like are known.
In such types, recently, a capacitive coupling type touch panel has been spotlighted. In a case where a touch is input to an input device using a button, a slider, or the like displayed on a display apparatus of a mobile electronic device, the input device needs to be disposed on the front surface of a display panel. In this case, an input function needs to be incorporated therein while maintaining a display image quality by minimizing a reduction in display luminance of the display apparatus. Here, in general, the resistive or the optical sensor type has a low transmittance of about 80%; however, the capacitive coupling type has a high transmittance of about 90%. Accordingly, the capacitive coupling type is advantageous in that the display image quality is not degraded. In addition, the resistive type senses a touch position by a mechanical contact of a resistance film. Therefore, the number of touches (mechanical contact) is increased, and the resistance film may be deteriorated or broken, so that there is a problem in that detection errors increase or detection failures may occur. On the other hand, the capacitive coupling type has no mechanical contact such as the contact of a detection electrode with other electrodes, and thus is advantageous in terms of durability.
As a capacitance detection circuit in the capacitive coupling type, for example, a type is disclosed in JP2005-140612 A. In the disclosed type, a sensor unit having a plurality of row wires and a plurality of intersecting column wires detects the capacitance existing in the vicinity of intersections thereof. When the pitch between the row wires and the column wires is reduced, fingerprint detection can be performed by detecting a change in capacitance that occurs due to unevenness of a surface of a fingerprint. On the other hand, as the sensor unit is made transparent and to have the same size as a screen of a display panel, a coordinate input device using a finger or the like as input means can be implemented. Capacitance detection is performed by applying a drive signal to the row wires sequentially selected from among the plurality of row wires from a row wire driving unit, and detecting a current flowing via the capacitance in the vicinity of an intersection between the row wire to which the drive signal is applied and the column wire, by using a capacitance detection circuit. Here, the capacitance detection circuit detects a capacitance on the basis of the difference of two detection current results. In the disclosed type, two means are disclosed for enhancing the capacitance detection precision by reducing external noise.
In the first means, from among column electrodes intersecting row electrodes that apply the drive signal, the column electrode which initially detects the capacitance in the vicinity of an intersection thereof compares the detected current to a reference current flowing through a reference capacitance of the capacitance detection circuits and calculates a capacitance, thereby enhancing the capacitance detection precision. The capacitance detection of the column electrode thereafter is performed by obtaining the difference between the detection currents of the adjacent column electrodes. In a case where capacitance detection of the row electrodes is performed after detecting the capacitances of all the column electrodes, the above-described operation is repeated.
In the second means, one is selected as a capacitance detection object from among the column electrodes to detect the detection current flowing through the corresponding column electrode, and the second detection current flowing through a plurality of the column electrodes excluding the corresponding column electrode is regarded as a reference current. Here, capacitances in the vicinities of the intersections thereof are detected by differences between the detection currents which are capacitance detection objects and the reference current.